This is the Apple Darwin open source SDK from the summer of 1999.
Included is source to Darwin 0.3 which corresponds to OS X Server 1.0. Source to the base operating system is included, along with binary images for early PowerMacintosh computers. The included disc image is a toast image, suitable for Macintoshes.
The Darwin 0.3 source is closer in lineage to NeXTSTEP, and OPENSTEP. This is before OS X 10.0 with the change from Mach 2.5 to 3.0, along with changing the drivers from using the Objective C driverkit, to the C++ IOKit.